In digital communication systems (e.g., high-speed optical communication systems) information sequences are transmitted, using a specific data protocol, in a form of repetitive structures referred to as “frames”. Such systems require synchronization between a transmitter and a receiver of the information. To accomplish such synchronization, the transmitter inserts a frame alignment sequence (FAS), typically at the beginning of a frame. In the receiver, the FAS allows to determine the position of the frame in the received digital stream. A frame synchronization module, referred to herein as “framer”, detects the FAS and monitors frame alignment once initial frame acquisition has been accomplished.
Typically, optical communication systems use an ON/OFF Keying (OOK) modulation format, and framers for such systems are known in the art. In the field of high-speed optical communication, Differential Phase Shift Keying (DPSK) and Duobinary Signaling modulation formats can offer significant advantages (e.g., a lower bit error rate) over the OOK format. To provide frame synchronization and determine the polarity of the received stream, these modulation formats require specialized frame synchronization algorithms. Conventional framers do not support multiple modulation formats. However, in a communication network, it is highly desirable to have framers which have an underlying algorithmic behavior that is independent from and cross-compatible with multiple modulation formats used by component digital communication systems.
Therefore, there is a need in the art for an improved method and apparatus for frame synchronization in digital communication systems using multiple modulation formats.